In Shemot 18:1, Yitro is identified as a priest of Midian and father-in-law of Moses. In verse 2 we are told again that he is the father-in-law of Moses. In verse 5 again. In verse 6, Yitro himself tells Moses "I am your father-in-law Yitro..." In verse 7 & 8, Yitro is refered to as 'his father-in-law', while in verse 9 & 10 he is called by name, Yitro. By verse 12 we again get both name & relationship. Verse 14 & 15 we hear relationship without name. This continues to the end of the chapter.
Even assuming we didn't know who Yitro was already (from Shemot 3:1), why mention this relationship over and over? Do we find in other instances of conversations between relatives that the relationship is mentioned over and over rather than their names? Also, here the relationship is only mentioned in one direction. We see no mention of Moses as 'his son-in-law'.